An extensibility model allows an application to enhance its functionality through extension code running in the application while providing a stable environment by sandboxing code that runs in each extension. For instance, in a web application, functionality of an application may be enhanced by running extension code in an iframe. When an extension is running in an application, the extension consumes resources (e.g., processing cycles, memory, network, etc.). When many extensions are bootstrapped and running, the resulting consumption of resources may degrade an experience of a user of the application. For instance, running a substantial number of extensions may reduce an operating efficiency and/or a reliability of the application and/or a computing device on which the extensions run.